Ozone, with its strong oxidizing power, is widely utilized in a great variety of applications, such as purification and sterilization of service water, treatment of waste water from industrial plants, treatment of sewage, denitration of waste gases and removal of different kinds of offensive odors.
Ordinarily, required to be used in the systems in which ozone gets involved, is excessive ozone for allowing the oxidation reaction to proceed completely. As a result, the unreacted ozone is discharged and exhausted.
As is well known, ozone is a substance attributed to air pollution or production of the so-called oxidants, and it badly damages the crops through destruction of chlorophyll, suppression of assimilation, etc., not to mention the adverse effects exerted by it on the human body. For that reason, and with a view to the prevention of the secondary environment pollution, excessive ozone must be removed. When air containing ozone at an increased concentration is fed into an airplane flying in the ozonosphere, there are caused several symptoms such as chest pain, cough and headache developing in crew members and passengers, which are furthermore likely to be accompanied by fatigue and lessened attentiveness, and it may be considered possible that the pilots will be kept from controlling and the safety of aerial navigation will be eventually jeopardized. For this reason it is earnestly desired to develop a highly efficient and economical method for treating waste ozone. So far proposed in the past have been (1) activated-carbon method, (2) thermal decomposition method, (3) absorption method with a chemical solution, etc.
The thermal decomposition method mentioned under (2) is a method comprising thermally decomposing ozone by means of a burner utilizing heavy oil, light oil, etc., and the method consisting of introducing an ozone-containing gas into a decomposition furnace at 300.degree. to 400.degree. C. to decompose ozone. The concentration of ozone in a gas is usually low, and the method has, therefore, the necessity of heating a large volume of air and is not economical.
The absorption method with a chemical solution mentioned under (3) is a method comprising introducing an ozone-containing gas into an aqueous solution of a reducing agent such as ferrous salt, sodium sulfite and sodium thiosulfate or an aqueous solution of alkali such as caustic soda to thereby absorb the ozone. Encountered in this method are problems, such as the troublesome procedures required for refilling to supplement the chemicals and treating waste solutions, a lowering of the absorption capacity due to a change in the composition of the chemical solution followed by absorption of ozone, inability to cope with and respond to the changing concentration of ozone in a gas (poor follow-up capability for fluctuating load of the ozone-containing gas), and treatment of waste solutions.
The activated-carbon method mentioned under (1) is a method comprising introducing an ozone-containing gas into a granular activated carbon layer to decompose the ozone to oxygen on the surfaces of the activated carbon. The method has the advantages for example, of being improved in the follow-up capability for a fluctuating load of the ozone-containing gas and of permitting ozone at a very low concentration to be decomposed at ambient temperature. However, it suffers disadvantages, such as a larger pressure loss in passage of air through the layer of activated carbon and the necessity of refilling to supplement a portion of the activated carbon lost through oxidation.
The decomposition mechanism of activated carbon by ozone, although being complicated, is meanwhile classified roughly into (1) action of ozone adsorption (in a narrow sense), (2) action of ozone decomposition catalysis, and (3) chemical reaction between oxygen molecules resulting from decomposition of ozone and activated carbon (combustion). The treatment of ozone with activated carbon has been conventionally considered to permit the treatment of about 5 g of ozone per 1 g of activated carbon, but the ozone decomposition mechanism with activated carbon can not be accounted for wholly by the stoichiometric relationship of three actions above-mentioned, because these three actions are mutually linked together in a complex way, and can be greatly affected by conditions of contact between activated carbon and ozone.
Our research study, while particularly paying our attention to the chemical phenomena at ambient temperature, led to the finding that at relatively low temperatures such as ambient temperature, the adsorption action of ozone onto activated carbon, most outstandingly, takes part in the ozone decomposition activity and the service life of activated carbon. That is to say, ozone is adsorbed by activated carbon at a fairly large rate in parallel with the catalysis. It is assumed that, when the adsorbed ozone is rapidly decomposed and released, the activated carbon, with its active sites being preserved, does not get deactivated. Yet, it has been found that complexes of C.sub.m O.sub.n (where m/n is 1 to 2) from carbon and oxygen are formed on the surfaces of activated carbon due to the powerful oxidizing property of ozone, and get gradually accumulated on the surfaces of activated carbon without being released, to cover the active sites, thus leading to a decrease in the ozone decomposition activity. Additional research studies, conducted based on such a novel finding, led the inventors to the finding that when one kind or not less than two kinds of alkali metals and/or alkaline earth metals are supported as the second component on the activated carbon supported with manganese compounds, ozone in a gas is very effectively decomposed on the surfaces of the resultant compositions, and the service life of activated carbon is prolonged up to three to four times that of the original one. On the basis of these findings, the present invention has been completed.